Tar sand is currently being exploited in the Athabasca region of Alberta by two large commercial plants. In general, these operations involve mining the tar sand, extracting the bitumen from the mined tar sand by the hot water process, and upgrading the recovered bitumen in a refinery-type circuit to produce synthetic crude oil.
The hot water process referred to is now well described in the literature. In summary, it comprises the steps of:
forming a hot aqueous tar sand slurry; PA0 conditioning the slurry by agitating it in a rotating horizontal drum, to effect a preliminary dispersion of the bitumen and solids and to entrain air bubbles in the slurry; PA0 screening the conditioned slurry, to remove oversize solids; PA0 diluting the conditioned slurry with additional hot water; PA0 introducing the diluted slurry into a thickener-like primary separation vessel and separating the greatest part of the bitumen from the solids by holding the diluted slurry for a period of time under quiescent conditions in said vessel, so that aerated bitumen may rise to produce overflow primary bitumen froth and solids may sink to produce underflow primary tailings; PA0 withdrawing a watery middlings stream from the midsection of the primary separation vessel, said stream containing fine solids and bitumen which was incapable of rising to the froth layer in the retention time allowed; and PA0 subjecting the middlings to vigorous aeration and agitation in a series of induced air flotation cells, to aerate bitumen and produce an overflow of secondary bitumen froth and an underflow of secondary tailings.
Further yields of froth may be obtained by induced air flotation performed on primary and secondary tailings.
It has long been recognized that the hot water process should be operated to maximize primary froth production and to minimize production of froth by induced air flotation. This is because the secondary-type froth is more heavily contaminated with solids and water than is the primary froth. Typically, primary froth contains about 60% by weight bitumen, while secondary froth only contains about 10-45% bitumen.
It has also long been understood that variations in the quality of the tar sand feed will affect the relative proportions of primary froth and secondary-type froths which are produced. More particularly, a tar sand low in bitumen content and high in fine solids content will produce a relatively small proportion of primary froth and a relatively large proportion of secondary froth, expressed as a percentage of the total bitumen in the feed. This result is attributed in part to the following. It appears that many of the flecks of bitumen in the `high fines` tar sand slurry are relatively small. These small flecks aerate relatively poorly and hence they are not as buoyant as would be desirable. Also, they seem to become associated with a proportionately larger amount of solids and thus their buoyancy is further deleteriously reduced. And finally, the `high fines` slurries tend to have a relatively high viscosity due to the high clay content--hence the aerated bitumen has difficulty in rising sufficiently quickly to reach the primary froth layer.
In any event, it is well recognized that it is desirable to improve the hot water process by increasing the proportion of the bitumen reporting as primary froth. This is particularly desirable in connection with the hard-to-process `high fines` slurries. It is to this end that the present invention is directed.
For purposes of the following description and the claims the term "secondary froth" is intended to encompass any froth produced by induced air flotation in connection with the hot water process--it is not to be limited to the secondary froth produced by induced air flotation of middlings from the primary separation vessel.